From Philip Lownman, "The libpng project decided to rename png_set_gray_1_2_4_to_8() to

png_set_expand_gray_1_2_4_to_8() with the 1.2.9 release.  This
submission fixes builds of the OSG against versions of libpng < 1.2.9
that don't have the new symbol available.  This affects platforms like
Red Hat Enterprise Linux 4 which come with libpng 1.2.7."

src/osgPlugins/png/ReaderWriterPNG.cpp

@@ -229,7 +229,14 @@ class ReaderWriterPNG : public osgDB::ReaderWriter
                 if (color == PNG_COLOR_TYPE_PALETTE)
                     png_set_palette_to_rgb(png);
                 if (color == PNG_COLOR_TYPE_GRAY && depth < 8)
+                {
+                #if PNG_LIBPNG_VER >= 10209
                     png_set_expand_gray_1_2_4_to_8(png);
+                #else
+                    // use older now deprecated but identical call
+                    png_set_gray_1_2_4_to_8(png);
+                #endif
+                }
                 if (png_get_valid(png, info, PNG_INFO_tRNS))
                     png_set_tRNS_to_alpha(png);